The present invention relates to a system for excuting an instruction in a digital computer, in particular, relates to a system for excuting "program call" instruction in a digital computer. The program call instruction is a novel instruction and facilitates the use of a program commonly in a plurality of program systems.
In a prior computer system, when a particular program is commonly used in a plurality of program systems, a particular software technique such as an open subroutine or a closed subroutin has been utilized.
In a closed subroutine system, any subroutine program must have a particular program step at the end of the subroutine for returning to a main program system.
And in an open subroutine system, a copy of a whole subroutine is transferred to another portion in a memory, so a large amount of memory area is necessary and further the coupling procedure of the copy program is necessary.
Accordingly, a prior sub-routine program which is to be commonly used in a plurality of main program systems must be designed as a sub-routine program having a particular program format, but an ordinary program which does not have a particular format, for instance, a return instruction at the end of the program, can not be commonly utilized.
A recent micro-processor or a micro-computer having a read-only memory has a plurality of programs in said read-only memory. However, those programs are not designed to be used in other program systems, so they of course do not have a return instruction.
As apparent from the above explanation, the disadvantage of the prior art is that a ready-made program which is not designed to be utilized as a sub-routine can not be used in other programs.